Microsoft uses ESE as an intermittent technology for moving the data from outlook client to database.

Eg: when outlook sends data, exchange acts as client and passes the data to ESE, now ESE accepts the data and move it to db.

Question:
Is there a similar concept ? if not how does client saves data to db

What does this have to do with ZImbra and what do you mean by this "... how does client saves data to db" - which DB? Perhaps if you gave a more clear explanation of what you're trying to achieve with Zimbra it might help.

Zimbra doesn't specifically store the mail in a DB, it stores them as plain old files and metadata is stored in MySQL. You'll find details of the system architecture in the documentation: System Architecture